A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such higher temperatures may cause increased nitrogen-oxide (NOx) emissions from the engine and may accelerate materials ageing, including turbocharger and exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When diluted air is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result.
EGR can also improve fuel economy in gasoline engines. At medium and high loads, fuel economy is improved due to knock mitigation, allowing for more efficient combustion phasing, reduced heat loss to the engine coolant, and lower exhaust temperatures—which in turn reduce the need for enrichment to cool the exhaust components. At low loads, EGR provides an additional benefit of reducing throttling losses.
In suitably configured engine systems, so-called ‘internal EGR’ may be used to achieve at least some of the advantages noted above. In this approach, combustion in one or more cylinders of the engine may be initiated when exhaust from a previous combustion is still present in the cylinders. The amount of internal EGR may be controlled using variable intake- and/or exhaust-valve timing.
To provide a higher level of intake-air dilution, ‘external EGR’ may be used instead of, or in addition to internal EGR. In this approach, exhaust gas discharged from the cylinder is routed back to the intake, where it mixes with fresh air. In boosted engine systems equipped with a compressor coupled to an exhaust-driven turbine, exhaust gas may be recirculated through a high pressure (HP) EGR loop and/or a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust gas is taken from upstream of the turbine and is mixed with intake air downstream of the compressor. In an LP EGR loop, the exhaust gas is taken from downstream of the turbine and is mixed with intake air upstream of the compressor.
HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. Moreover, each strategy presents its own control-system challenges. For example, HP EGR is most effective at low loads, where intake vacuum provides ample flow potential; at higher loads, the desired EGR flow rate may be unattainable due to reduced flow potential. Intrinsically dependent on turbocharger waste gate and throttle conditions, HP EGR may require a complex flow-control strategy. Further, HP EGR may suffer from poor EGR/intake-air mixing and may require a high rate of active cooling due the short length between the HP EGR take-off point and the intake runners of the engine.
In contrast to HP EGR, LP EGR provides adequate flow from mid to high engine loads (areas where HP EGR may be flow limited), is more easily cooled, and can be controlled more independently of the throttle and waste gate. However, LP EGR may respond sluggishly to changing engine load, engine speed, or intake air flow. In gasoline engines especially, such unsatisfactory transient response may result in combustion instability during tip-out conditions, when fresh air is needed to sustain combustion but EGR-diluted air is present upstream of the throttle valve. Moreover, a significant lag in EGR availability can occur during tip-in conditions, as the amount of EGR accumulated in the intake manifold may not be sufficient to provide the desired combustion and/or emissions-control performance.
Turbocharged engine systems using more than one EGR mode have been described. For example, World Intellectual Property Organization Patent Application Publication Number 2007/136142 describes a system wherein a ratio of internal and external LP EGR is adjusted depending on engine operating conditions. However, this reference does not contemplate the full range of control options that are possible when fast-responding internal EGR is coordinated with slower-responding external LP EGR.
Therefore, one embodiment provides a method for controlling combustion in a cylinder of a turbocharged engine in which intake air is reserved upstream of the cylinder. The method comprises decreasing an internal EGR rate in the cylinder during a tip-out condition if the temperature of the intake air is above a threshold, and increasing the internal EGR rate in the cylinder during a tip-out condition if the temperature of the intake air is below the threshold. By applying divergent strategies to enhance combustion stability during tip out, depending on the intake-air temperature, various benefits are realized. Such benefits may include extending the steady-state operating range over which cooled LP EGR may be used, while protecting combustion stability during transients.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.